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Mesoscale Discussion 79
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   Mesoscale Discussion 0079
   NWS Storm Prediction Center Norman OK
   0209 PM CST Fri Jan 20 2017

   Areas affected...portions of the middle and upper Texas coast

   Concerning...Severe potential...Watch possible 

   Valid 202009Z - 202215Z

   Probability of Watch Issuance...40 percent

   SUMMARY...Isolated thunderstorms will continue to develop into the
   late afternoon and evening across the middle and upper Texas coast.
   Additional development eventually is expected across eastern Texas,
   but timing of more widespread convection is a bit uncertain. 
   Initial concerns this afternoon will be hail and some strong wind

   DISCUSSION...Isolated thunderstorms were developing along the middle
   TX coast near VCT, in the vicinity of a warm front draped northeast
   to southwest across the coastal plain. Weak destabilization is
   occurring as temperatures warm into the upper 60s to upper 70s with
   weak capping eroding. Latest hi-res guidance indicates coverage
   should remain rather isolated for a few hours, before a gradual
   increase occurs into the evening hours. This seems reasonable given
   a weak shortwave impulse is currently moving across the TX Big Bend
   area and will provide increasing forcing for ascent in the coming

   In the meantime, isolated strong to severe thunderstorms will be
   possible during the next few hours in the vicinity of the surface
   boundary. Forecast RAP soundings coupled with mesoanalysis data show
   steep low and midlevel lapse rates improving between now and 00z. At
   the same time, increasing vertical shear, with long hodographs
   suggest large hail will be the main concern as storm track along to
   just north of the surface boundary. Some backed low level flow in
   the vicinity of the boundary will lead to some enhanced shear in the
   lowest 1 km or so, posing some low-end tornado potential. However,
   wind speeds less than about 10-15 kt in this layer will limit
   greater tornado threat. With low level lapse rates increasing to 7-8
   C/km, a few strong wind gusts also will be possible. While coverage
   is expected to remain limited in the short-term, a watch may be
   needed in the next 1-3 hours, depending on convective trends.

   ..Leitman/Hart.. 01/20/2017


   LAT...LON   27769711 27769751 28059795 28569794 29479692 30249578
               30519507 30579444 30489405 30319385 29749376 28829522
               28099661 27769711 

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